(1) Field of the Invention
The present invention relates to a mask for polycrystalline silicon fabrication processes, especially to a sequential lateral solidification (SLS) mask which could be used for crystallizing an amorphous silicon film onto a multi-boundary polycrystalline silicon.
(2) Description of the Prior Art
Thin film transistors (TFTs) are widely applied to liquid crystal display (LCD) devices. In the art, two materials, the amorphous silicon (a-Si) and the polycrystalline silicon (poly-Si), are known to be two of major materials for forming the TFTs.
The polycrystalline silicon has lots of characters and is superior to the amorphous silicon while being used in the LCD. For instance, the polycrystalline silicon has bigger grains so that the speed of the response time of the TFT using the polycrystalline silicon is faster than that using the amorphous silicon. Furthermore, a poly-Si TFT LCD can use a backlight module to lower its power consumption.
Low Temperature Polycrystalline silicon (LTPS) process is a common process to crystallize the amorphous silicon. The LTPS usually uses Excimer laser as its heat source. While the laser beam irradiates onto an amorphous silicon film, the amorphous silicon film absorbs energy of the laser beam and is transformed into a polycrystalline silicon film.
One method of crystallizing the amorphous silicon into polycrystalline silicon is a sequential lateral solidification (SLS), which using a technique of Excimer laser annealing. SLS crystallization uses the fact that silicon grains tend to grow laterally from the boundary of the liquid silicon and the solid silicon. With the SLS, the amorphous silicon is crystallized by using a laser beam having a sufficient magnitude and a relative motion to melt the amorphous silicon, such that the melted silicon can grow laterally silicon grains in recrystallization.
Please refer to FIG. 1A, which is a schematic cross-section view showing how a poly-Si film is fabricated by the SLS. As shown, an amorphous silicon film 11 formed by a chemical vapor deposition or sputtering is located on a substrate 10, and a mask 2 is disposed above the amorphous silicon film 11. The mask 2 comprises a plurality of light transmitting slits 21 and light absorptive slits 22. Please refer to FIG. 1B, a laser beam irradiates the mask 2 along the direction shown by arrows in FIG. 1A with a back-and-forth scanning pattern of stroking horizontally but feeding vertically (as a figure view of FIG. 1B). After the amorphous silicon film 11 is well irradiated, a lateral-growth poly-Si is then formed.
Please refer to FIG. 1C, which shows the pattern formed by the above process via the mask 2. The grain boundary 111 in FIG. 1C is perpendicular to the growing direction of the grains. According to various tests, channels of the TFTs could have better electrical performance while they are arranged parallel to the growing direction of the grains. For the above reason, almost all channels of the TFTs in the art have been designed parallel to each other, and such arrangements restrict the design variety in circuit patterning.
Therefore, how to remove the limitation caused by the grain-growing direction is necessary and important.